It did not have sprinklers or firewalls installed, like a modern building might. "There had been a systematic refusal to install anything electrical" because of the risk of fire, Pierre Housieaux, president of the Paris Historical Association, told the New York Times.

From the attic, the fire spread through the roof toward the spire. Google Earth imagery shows the challenge firefighters faced when trying to tackle the blaze.

But the exterior is covered with art. Countless grotesques and gargoyles — ornate characters used as much for dispersing rain water as for decoration — as well as the building's renowned stained-glass windows were all at risk from the flames.

The three largest rose windows measure up to 19 metres in diameter. Notre Dame's heritage director Laurent Prades said they were not destroyed but may have been damaged by the heat.

The flames spread towards the cathedral's two iconic towers — for a long time the tallest structures in Paris — and at one point, as the fire burned, it seemed like the entire cathedral might perish.

Notre Dame has been at the centre of history for almost a millennium, from Joan of Arc to the Nazi occupation — yet now the end seemed near, watched in real-time by millions around the world.

It could have left a smouldering ruin on a site that is the literal heart of Paris, the place where all distances are measured from. Instead the towers were saved by firefighters, about 500 of whom battled the blaze.

Designed in the 12th century using a modern technique for the time, it may have been a factor in keeping the ceiling intact while the roof burned.

Notre Dame's six-part vaults, an early example of the style in Europe, were designed like an arch, allowing for high ceilings — letting more light into the nave — while remaining structurally sound.

"Most of the vaults seem to have survived, which is a testament to how well they were put together in the 12th century," Dr Robert Bork, an architectural historian at the University of Iowa, told the ABC.

While money to restore the cathedral has been flowing in, it was built using methods of medieval artisanry that may not be perfectly replicated, and some of the wood that burned may be replaced by other materials.

"You might say, 'well it's not actually recreating Notre Dame as it was. But perhaps it can't be recreated as it was'," Professor Peter McPhee, a specialist in French history at the University of Melbourne, told the ABC.

As shocking as it was to witness, in time this episode will become just another fold in the fabric that makes up the grand old dame of Paris — to take its place alongside the stories of saints and martyrs, kings and hunchbacks.